Average δ18O Research Articles

Assessing inter-basin groundwater input to the Verlorenvlei estuarine lake using stable isotopes and hydrochemistry

Study region: Verlorenvlei Catchment (∼1 890 km2) is an agriculture-dominated area (∼43 % per km2) on South Africa’s west coast. This semi-arid region has variable rainfall and high evaporation rates, affecting the three major aquifers and Verlorenvlei – a RAMSAR-listed estuarine lake. Study focus: Natural processes (i.e., extended dry periods and evaporation) and anthropogenic activities (i.e., agricultural expansion and groundwater abstraction) have threatened Verlorenvlei’s ecological functions. Seasonal and spatial changes between the water sources (i.e., direct rainfall, surface water, and groundwater) supporting Verlorenvlei were determined using δ18O and δ2H isotopes and hydrochemical analyses. Inter-basin aquifer contribution was investigated to assist in explaining Verlorenvlei's slow recovery since the recent 2015 – 2018 Western Cape drought. New insights: A proportion of groundwater from outside the topographic and surface-water delineated catchment supports Verlorenvlei during the dry month of April (i.e., G30F Langvlei sub-catchment). Furthermore, Verlorenvlei experiences high evaporation (evaporation best fit line: δ2H = 12.49 x δ18O - 47.68, average δ2H value of 47.1 ‰ and average δ18O value of 7.64 ‰) compared to its feeding rivers. Two sandstone and shale-dominated sub-catchments exhibit overlapping groundwater δ18O and δ2H values and water types to the sub-catchment in the nearest vicinity of Verlorenvlei, suggesting a disproportionately high groundwater contribution from these sub-catchments to Verlorenvlei. Evaluation of Verlorenvlei’s water balance should consider both surface water and groundwater sources, particularly from inter-basin aquifer sources during prolonged droughts.

Temporal variation in oxygen isotopes of peraluminous granites derived from sedimentary sources

The oxygen isotopic composition of magmatic rocks provides critical constraints on the contributions of crust versus mantle in their genesis. Peraluminous granites derived from partial melting of sedimentary rocks offer a unique archive to examine how the oxygen isotope composition of sedimentary rocks recycled into magmas has changed throughout Earth history. In this study, we explore this record through a two-pronged approach. First, we compile a comprehensive dataset of oxygen isotope bulk-rock and mineral analyses from globally distributed peraluminous granites and volcanic rocks ranging in age from 3.2 billion years to 6 million years. Second, we model the isotopic fractionation between sedimentary source rocks and derivative melts, as well as minerals in equilibrium with the melt during anatexis. Our compilation demonstrates a progressive increase in δ18O values of zircon and garnet from the Archean to Paleoproterozoic and again in the latest Neoproterozoic to Phanerozoic. Specifically, the average δ18O of zircon from peraluminous granites increases from 6.7 ± 1.0 ‰ to 9.5 ± 0.8 ‰ at ∼ 2.35 Ga, followed by another increase to 10.4 ± 0.7 ‰ after 600 Ma (errors are ± 1 s.d.). These observations align with the sedimentary rock record which suggests enhanced weathering and clay deposition resulting from tectonic and biological forcings. Furthermore, the increase in δ18O of peraluminous granites broadly mirrors similar secular increases in the δ18O values of siliciclastic sedimentary rocks. However, using our modeled fractionations between melt and source rock, the reconstructed source-rock δ18O values capture only the lower end of the siliciclastic sedimentary rock range, suggesting that sedimentary rocks recycled into magmas have, on average, lower δ18O values than coeval clay-rich shales. We propose that any global analyses of oxygen isotopes in magmatic rocks or zircon should use the average values of calculated source rocks for peraluminous granites in defining crustal contributions. Furthermore, our study highlights the broader implications of the observed trends, including the progressive contamination of the mantle and elevation of its δ18O values.

A Tale of Two Storms: Inter‐Storm Variability of Stable Water Isotopes in a Solute Transport Model

ABSTRACTStable isotopic methods in hydroclimate monitoring are powerful for improving water resources management, but applications are limited, especially in semi‐arid regions where such management is needed most. Here, we show that we can address shortcomings related to the lack of a seasonal signal using stable water isotopic signatures measured in precipitation over the East San Francisco Bay area, California, during two contrasting events sampled at more than 20 locations in the winter of 2023. The observed range in δ18O in the rain samples is similar for both storms. However, the distributions do not overlap—the mean air temperature and δ18O during Winter Storm Olive (February 2023) were 2°C and − 12‰, respectively, while a warm atmospheric river event (March 2023) had a mean temperature of 9°C and δ18O of −6‰, close to the long‐term average δ18O measured in local precipitation. The Winter Storm showed expected trends in δ18O related to geography (i.e., lower with greater distance inland and elevation), while the atmospheric river δ18O pattern was more spatially uniform. We use hydrometric data from a gaged watershed in the study area and isotopic signatures of rain sampled during the two storm events and apply a solute transport model (StorAge selection) with a travel‐time approach to examine predicted watershed responses and potential water tracing applications. In this virtual experiment, we find that event size exerts a strong control on the relative amounts of runoff versus pre‐event water in the stream, while uncertainty in stream hydrograph separation is related to the degree of contrast between precipitation/runoff and pre‐event water. Key to flood prediction, adaptation, and mitigation, especially in coastal urban areas, is knowledge of the contributing water sources and timing of stream flow. The strong contrast in stable isotopes between these two events, close in time and over the same area, illustrates the potential to use stable isotope signatures to track the transport and mixing of events through natural and engineered watersheds that are threatened by climate whiplash events.

Corundum ± magnesium‐deltalumite ± hibonite‐bearing objects in the CH chondrite Sayh al Uhaymir 290

AbstractIsolated corundum grains and corundum ± Mg‐deltalumite [(Al,Mg)(Al,◻)2O4] ± hibonite assemblages were investigated in the CH3.0 metal‐rich carbonaceous chondrite Sayh al Uhaymir (SaU) 290. Although very refractory inclusions containing abundant Zr‐ and Sc‐rich oxides and silicates, hibonite, grossite, or perovskite have been previously described in CH chondrites, this is the first discovery of corundum and Mg‐deltalumite in CHs and the first discovery of Mg‐deltalumite in nature. Magnesium‐deltalumite can be indexed by the Fd3m spinel‐type structure and gives a perfect fit to the synthetic Al‐rich spinel cells. Corundum‐Mg‐deltalumite grains, 5–20 μm in size, are occasionally rimmed by a thin layer of hibonite replacing corundum. Some corundum grains contain tiny inclusions of ultrarefractory Zr,Sc‐rich minerals and platinum‐group element (PGE) nuggets. All corundum, hibonite, and Mg‐deltalumite grains studied have 16O‐rich compositions (average Δ17O ± 2SD = −22 ± 3‰). Two corundum grains show evidence for significant mass‐dependent fractionation of oxygen isotopes: Δ18O ~ +34‰ and ~ +19‰. We suggest that the SaU 290 corundum‐rich objects were formed by evaporation and/or condensation in a hot nebular region close to the proto‐sun where the ambient temperature was close to the condensation temperature of corundum. A corundum grain with tiny inclusions of Zr‐ and Sc‐rich phases and PGE metal nuggets recorded formation temperatures higher than the condensation temperature of corundum. Two corundum‐rich objects with highly fractionated oxygen isotopes must have crystallized from a melt that experienced evaporation. Corundum grains corroded by hibonite recorded gas–solid interaction in this region during its cooling. The Mg‐deltalumite ± corundum ± hibonite objects were formed by rapid crystallization of high‐temperature (>2000°C) refractory melts. The lack of minerals with condensation temperatures below those of corundum and hibonite in the SaU 290 corundum‐rich objects suggests that after formation, these objects were rapidly removed from the hot nebular region by disk wind and/or by turbulent diffusion and disk spreading.

Groundwater evolutions in the semi-arid aquifers of Dodoma City, Tanzania using stable isotopes of water and hydrogeological facies

ABSTRACT The recent influx of people into Dodoma City, Tanzania has greatly increased pressure on freshwater sources. To manage such stressed water resources, an assessment of evolutions of various groundwater masses using stable isotopes (SI) and hydrogeochemical facies was performed. The major groundwater types were found to be NaHCO3 and NaCl. The groundwater in some areas was polluted with up to 223.7 ppm of nitrate-NO3-. The majority of water masses were found to be of meteoric origin and deep aquifers were mostly replenished by heavy rains, which are more depleted in SI (δ2H and δ18O). Further, δ2H and δ18O from Hombolo Dam (HD) and Matumbulu Dam (MD) were highly enriched by evaporation, with average δ2H and δ18O values, of 5.76 & 39.69 and 2.37 & 16.34 ‰ compared to average rain values of −5.52 and −32.99‰, respectively. The contributions of highly enriched HD and MD waters to respective shallow groundwater systems were 57.52 and 22.2%. Yet, it was found that 75.4% of groundwater in the Hombolo suburb originates from the Makutupora aquifer, and this is the first time the contribution is quantified. Generally, this study offers a robust tool for adapting a local groundwater management strategy impacted by climate change, pollution, and overabstraction.

Spatial ecology of moose in Sweden: Combined Sr-O-C isotope analyses of bone and antler.

The study of spatial (paleo)ecology in mammals is critical to understand how animals adapt to and exploit their environment. In this work we analysed the 87Sr/86Sr, δ18O and δ13C isotope composition of 65 moose bone and antler samples from Sweden from wild-shot individuals dated between 1800 and 1994 to study moose mobility and feeding behaviour for (paleo)ecological applications. Sr data were compared with isoscapes of the Scandinavian region, built ad-hoc during this study, to understand how moose utilise the landscape in Northern Europe. The 87Sr/86Sr isoscape was developed using a machine-learning approach with external geo-environmental predictors and literature data. Similarly, a δ18O isoscape, obtained from average annual precipitation δ18O values, was employed to highlight differences in the isotope composition of the local environment vs. bone/antler. Overall, 82% of the moose samples were compatible with the likely local isotope composition (n = 53), suggesting that they were shot not far from their year-round dwelling area. 'Local' samples were used to calibrate the two isoscapes, to improve the prediction of provenance for the presumably 'non-local' individuals. For the latter (n = 12, of which two are antlers and ten are bones), the probability of geographic origin was estimated using a Bayesian approach by combining the two isoscapes. Interestingly, two of these samples (one antler and one bone) seem to come from areas more than 250 km away from the place where the animals were hunted, indicating a possible remarkable intra-annual mobility. Finally, the δ13C data were compared with the forest cover of Sweden and ultimately used to understand the dietary preference of moose. We interpreted a difference in δ13C values of antlers (13C-enriched) and bones (13C-depleted) as a joint effect of seasonal variations in moose diet and, possibly, physiological stresses during winter-time, i.e., increased consumption of endogenous 13C-depleted lipids.

VARIATIONS OF δ18О AND δ2H VALUES OF PRECIPITATION IN MOSCOW FROM 2017 TO 2019

To reveal variations in the iso topic composition of O and H in the atmospheric precipitation in Moscow and the processes influencing the isotope composition, all events of precipitation in 2017-2019 were sampled at the Meteorological Observatory of the Moscow State University: 158 samples in 2017, 119 samples in 2018 and 143 samples in 2019. The study is a prolongation of continuous measurements of the isotope composition of precipitation, started by authors in 2014. The study of the isotope composition of precipitation at the MSU Meteorological Observatory was supported by the IAEA and became a part of the Global Network of Isotopes in Precipitation (GNIP) database. It has been found that the intra-annual variability of the isotop e composition of precipitation has a pronounced seasonality. The most isotopically heavy precipitation falls from May to August, and the most isotop ically light precipitation at December-February, mainly due to seasonal air temperature variations. The ratio of the average monthly δ18O values in precipitation and air temperature for the study period varied from 0.34 to 0.39‰/°C, which is consistent with the previously obtained data for precipitation in Moscow. The δ2H-δ18O ratio in precipitation was clos e to that of the Global Meteoric Water Line, pointing to the equilibrium conditions during precipitation formation. It was established that in the summer months isotopic composition is significantly influenced by undercloud evaporation. The deuterium excess va lues in precipitation are not markedly seasonal; however, lower dexc values (below the 3-year average of 11‰) are typical for the summer months (July-August). It is most likely due to undercloud evaporation in conditions of low relative humidity and high air temperatures. Higher dexc values (above 11‰) prevailed from October to April.

Low δ18O and δ30Si TTG at ca. 2.3 Ga Hints at an Intraplate Rifting Onset of the Paleoproterozoic Supercontinent Cycle

AbstractThe start of the Paleoproterozoic supercontinent cycle is typically taken as the initiation of orogenesis at ca. 2.1 Ga leading to the assembly of Earth's first supercontinent, Columbia. However, the dearth of ca. 2.5–2.2 Ga geological records makes it difficult to deduce tectonic factors during the onset of the Paleoproterozoic supercontinent cycle. The petrogenesis of tonalite–trondhjemite–granodiorite (TTG) provides useful proxies for tracing prevailing geodynamic regimes of early continental evolution. However, marked decreases of TTG and other magmatism occurred across the Archean–Paleoproterozoic transition and have previously precluded forming testable hypotheses. Early Paleoproterozoic TTGs have been identified in the North China Craton (NCC) and other cratons, which may represent the last major pulse of TTGs globally. Here we present low δ18O and δ30Si ca. 2.3 Ga TTGs from the NCC, together with thermodynamic modeling and compilation of stable O and Si isotopes for TTGs globally through time. The ca. 2.3 Ga TTGs were derived from the partial melting of Archean basaltic crust and give lighter average zircon δ18O (3.15 ± 0.35‰) and whole‐rock δ30Si values (−0.17 ± 0.08‰) than most Archean TTGs. Considering coeval mafic‐felsic igneous rocks, and lithospheric thinning since ca. 2.5 Ga based on estimated crustal thickness through the Neoarchean–Paleoproterozoic, we posit the onset of an intraplate rifting consistent with the anomalous low‐δ18O magmatism. Continental rifting of Archean cratons/supercratons plausibly hints at the formation of rifts driving subduction initiation as the veritable onset of the Paleoproterozoic supercontinent cycle.

The transmission of isotopic signals from precipitation to groundwater and its controls: An experimental study with soil cylinders of various soil textures and burial depths in a monsoon region

The use of stable isotopic composition in groundwater as a paleoclimatic proxy assumes that the isotopic values in groundwater are inherited from the corresponding values in the source precipitation. However, several studies have observed that groundwater δ18O (δ18Og) is not always in accordance with the average δ18O of precipitation (δ18Op). The cause of this inconsistency is still the subject of debate. Significant reduced isotopic variability and potential deviations from precipitation to groundwater because of the aquifer itself (e.g., groundwater buried depths and soil texture) and external precipitation characteristics (e.g., rainfall intensity and duration) have been poorly quantified in terms of identifying the dominant factors. To investigate the controls on the transmission of isotopic signals from precipitation to groundwater, we present new stable isotope data for precipitation and groundwater from an experimental site using eight soil cylinders of different soil textures and burial depths in a monsoon region. We collected data at inter-event sampling intervals (30 min) during and after two heavy precipitation events (daily amounts higher than 10 mm). Our results demonstrated a significant disparity between the average isotopic values of heavy rainfall and light rainfall, despite both exhibiting decreasing isotopic patterns throughout the course of the rainfall event. Various degrees of attenuation of isotopic variations were found in outflow waters with different groundwater burial depths (GBDs) and soil textures after rainfall events. The isotopic variations were not synchronized with the outflow changes. Specifically, there were evident outflow lags in response to heavy rainfall, especially in the lysimeters with GBDs deeper than 0.5 m, while the greatest isotopic variation was observed in the early infiltrating stage. Redundancy analysis indicated that precipitation amount was the most important contributor to the outflow variation (∼51 %, p < 0.05), while GBD was the key factor for isotopic temporal variation (∼52 %, p < 0.05). These results concurred with the clustering of principal component analysis results. The mixing of soil water and heavy rainfall event components was observed in four lysimeters with mixing proportions fP varying in the range of 7 %–51 %, indicating the effective recharge from heavy rainfall. Soil evaporation was highlighted with samples located parallel to the local meteoric water line. It is recommended that groundwater isotopes in paleoclimatic studies are used with additional constraints that consider disproportionate recharge from intense rainfall, past chaotic fluctuations in groundwater dynamics, and distortions introduced by evaporation.

Holocene stable oxygen isotope record from the North Caspian Rybachya core

The first results of 22 δ18Oost measurements on ostracod shells from the Rybachya core in the North Caspian Sea are presented. The stable oxygen isotope record here confirms previous studies of Upper Quaternary and Holocene sediments of the Rybachya core area, where it was suggested that a paleovalley was formed during the Mangyshlak regressive stage and then gradually filled during the Holocene transgressive stage. We recorded three local isotopic stages, each characterized by δ18Oost values. The hydrological variability and sea level changes associated with the transgressive-regressive rhythm of the Caspian Basin and the climatic changes in the Northern Hemisphere led to the δ18Oost changes obtained in this work. Relatively depleted δ18Oost values from sediments filling the paleovalley allowed to interpret the isotope record in terms of freshwater influx. The average δ18O enrichment of ostracod shells is taken as evidence for the isolation of the paleovalley during the Caspian Sea level retreat. However, the higher δ18Oost values obtained for the last (modern) stage of Caspian history are under the strong control of host water salinity, which is higher than during the paleovalley filling episode.

Determining the source water and active root depth of woody plants using a deuterium tracer at a Savannah site in northern Stampriet Basin, Namibia

Woody plants play a significant role in the global water cycle through water uptake by roots and evapotranspiration. A deuterium tracer was used to assess the active root depths for Salvia mellifera and Boscia albitrunca in the Ebenhaezer area (western Namibia). The tracer was inserted at different soil depths in December 2016. Xylem cores were obtained using an increment borer, and transpired water was collected using transpiration bags zipped around the plants’ leaves. Groundwater was collected from boreholes. Soil samples were collected after the rainy season using a hand auger. Xylem and soil water were extracted using a cryogenic vacuum extraction method and analysed for stable water isotopes. Only one S. mellifera transpiration sample showed a high deuterium content (516‰) where the tracer was inserted at 2.5-m soil depth. Elevated deuterium contents were observed in two S. mellifera xylem samples; tracer had been applied at 2.5 and 3 m depth (yielding 35 and 31‰ deuterium, respectively), which constitutes a possible active-root depth range for S. mellifera. At the end of the study period (May 2017), the average δ18O value for B. albitrunca xylem samples was similar to that of groundwater. The δ18O value for S. mellifera was between that of soil water and groundwater, indicating that this species uses groundwater and soil water available for groundwater recharge. Determination of the active root depth and source water for these species would help improve hydrological modelling by incorporating the influence of woody plants on groundwater recharge.

Tracing fluid infiltration into oceanic crust up to ultra-high-pressure conditions

Fluid–rock interaction within the altered oceanic crust and across the slab–mantle boundary during subduction facilitates element transfer, but the dynamics of fluid transport and fluid–rock exchange during upward fluid migration are still unclear. A study of metamorphic fluid–rock interaction within a section of subducted oceanic crust was carried out on eclogites and metasediments of the ultra-high-pressure Lago di Cignana Unit (NW Italian Alps). The P–T modeling of a quartzschist shows that garnet grew during the prograde and sporadically during the retrograde path and that phengite mainly records the peak to retrograde conditions. Microscale geochemical analysis of garnets has revealed a systematic evolution of oxygen isotopic composition with garnet major element zonation, with extreme within-sample core–rim variations in δ18O between 18 and 4‰ providing evidence for external fluid influx. Garnet in eclogites and calcschists, as well as garnet cores in quartz-rich lithologies, shows normal compositional zoning, as expected for prograde garnet growth, and a relatively constant oxygen isotopic composition. The outer garnet growth zones within a few metasediments show reverse or discontinuous zoning and progressively lower δ18O. Despite major element zoning, the isotopic composition of mica is homogeneous across chemical zoning in one eclogite and one quartzschist, but shows 6‰ variability in another quartzschist. In the underlying Zermatt–Saas serpentinites, antigorite from nine serpentinite samples shows some variation in δ18O, with average δ18O values for individual samples ranging from 1 to 6‰. These results provide evidence for two main stages of external fluid infiltration: (i) fluids from the dehydration of mafic lithologies entered the sequence at peak conditions around 3 GPa, as indicated by the oxygen composition of intermediate zones of mica and garnet, and (ii) low δ18O fluids from serpentinites infiltrated parts of the sedimentary package during exhumation prior to 1.5 GPa, as recorded by the 4‰ garnet outer rims. Samples recording external fluid infiltration are concentrated in the lower part of the sequence, indicating channelized fluid flow, suggesting focused fluid infiltration due to permeability contrasts between metasedimentary and eclogitic lithologies. Channelized fluid flow in the ultra-high-pressure metasediments of Lago di Cignana has not resulted in systematic decarbonation of the metasediments.

Exploring the Deuterium Excess of Cretaceous Arctic Paleoprecipitation Using Stable Isotope Composition of Clay Minerals from the Prince Creek Formation (Maastrichtian) in Northern Alaska

We report estimated stable isotope compositions of Artic paleoprecipitation using phyllosilicates sampled from three paleosols and two bentonites in the Prince Creek Formation (Maastrichtian) in northern Alaska. Previous studies reported a deuterium excess in estimates of Arctic paleoprecipitation from the Late Cretaceous by combining hydrogen and oxygen proxy sources, including pedogenic minerals, dinosaurian tooth enamel phosphates, pedogenic siderites, and n-alkane biomarkers. The new dataset produced in this study removes uncertainty on possible explanations (photosynthetic and transpiration) of the deuterium excess by producing stable hydrogen and oxygen isotopic signatures from the same source material. The δD of the phyllosilicates range from −171‰ to −72‰ VSMOW and δ18O ranges from 5.0 to 11.8‰ VSMOW. By assuming a MAT of 6.3 °C and calculating uniquely derived fractionation equations for each phyllosilicate, we report estimated isotopic composition of Late Cretaceous paleoprecipitation with an average δD value of −133‰ VSMOW, corresponding to an average δ18O value of −20.3‰ VSMOW. The estimates of Late Cretaceous paleoprecipitation do not intersect the Global Meteoric Water Line and reveal a reported deuterium excess ranging from 7 to 46 per mil. These results confirm the presence of a deuterium excess in Late Cretaceous Arctic paleoprecipitation and provide new insight to assessing possible explanations for this phenomenon.

Assessing the Applicability of Mainstream Global Isoscapes for Predicting δ18O, δ2H, and d-excess in Precipitation across China

Precipitation isoscapes have provided supporting data for numerous studies of water stable isotopes, alleviating the lack of observation data. However, the applicability of simulation data from global models to specific regional contexts remains a subject requiring further investigation, particularly concerning d-excess—an aspect often overlooked by prediction models. To bridge this gap, this study evaluates the performance of three mainstream precipitation isoscapes (OIPC3.2, RCWIP1, and RCWIP2) for the prediction of average annual δ2H, δ18O, and d-excess based on observations from the CHNIP database. The results show that while all three models can accurately reproduce δ2H and δ18O values, none are able to accurately match d-excess values. This disparity can be attributed to the absence of water-vapor source information in the models’ input variables, a key determinant influencing d-excess outcomes. Additionally, it is noteworthy that OIPC3.2 stands out as the optimal choice for δ2H and δ18O estimations, while RCWIP2 exhibits progressive enhancements over RCWIP1 in d-excess estimations. This highlights the significance of selecting highly pluralistic information variables and recognizing the impact of error propagation in such models. As a result, the advancement of isoscapes in accurately and precisely depicting precipitation isotopes, particularly d-excess, necessitates further refinement. Future avenues for improvement might involve the incorporation of water-vapor source-clustering methodologies, the selection of information-rich variables, and the autonomous construction of a dedicated d-excess simulation. This research provides valuable insights for the further refining of isoscape modeling in the future.

A High-δ18O Mantle Source for the 2.06 Ga Phalaborwa Igneous Complex, South Africa?

ABSTRACTThe 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.5 and 8.1‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O values that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from −79 to −48‰ with a mean of −64 ± 9‰ (1σ, n = 19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid–rock interaction in the carbonatite–foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, −3.19 to −5.60‰) overlaps with the unaltered, primary igneous carbonatite field.Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n = 8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n = 5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.

A Seasonally Varying Tree Physiological Response to Environmental Conditions: Results From Semi‐Arid China

AbstractDrought is the primary limiting factor for tree growth in water‐stressed regions through its effects on xylem development. In this study, we developed intra‐annual series of tree‐ring stable carbon (δ13C) and oxygen (δ18O) isotopes from Chinese pine (Pinus tabuliformis) to investigate the ecophysiological responses of trees to drought during the growing season in a climatic transition zone between arid and semi‐arid areas in north‐central China. Distinct intra‐annual patterns of δ13C and δ18O were revealed, with average annual amplitude of 0.26 and 2.72‰, respectively. Tree‐ring δ13C values increased from the early‐ to mid‐growing season, peaked in the mid‐to‐late‐growing season, and decreased at the end of the growing season. In contrast, δ18O values continuously decreased throughout the growing season. As expected, drought significantly increased δ13C and altered its intra‐annual pattern, particularly toward the end of the growing season. However, there was no significant difference in the average intra‐annual pattern in δ18O between dry and wet years. The tree‐ring δ18O at the end of the growing season was significantly correlated with the September to October average soil water δ18O, as simulated by a global isotope‐enabled climate model, which is consistent with the high proportion of exchangeable oxygen atoms predicted by previous studies for cellulose synthesis in the late growing season.

Temporal variability of precipitation isotopes in Damascus, Syria — implications for regional climate change

Records of stable isotopes (2H and 18O), deuterium excess (d-excess) and tritium (3H) values in precipitation (P) during 1990–2022, together with long-term time series (1919–2022) of air temperature (T) and P-amount values at the Damascus station, were analysed to explore the seasonal and annual variability patterns of those parameters in P and assess the vulnerability to climate change in this area. Variation of the annual average air T values over the period 1919–1969 shows an increase gradient of ≈ + 0.1 °C/decade. However, a remarkable much higher gradient (+ 0.64 °C/decade) is calculated for the period 1990–2022. The average P-amount value calculated for the last three decades (≈185 mm) was lower by ≈28 mm, compared to the value (≈213 mm), relative to the reference period (1919–1969). This significant decline in the annual P-amount value by ≈11–13%, accompanied by an annual heating of 0.2–0.6 °C/decade in the annual air T, is likely the result of the climate change affecting this area. The linear relationships between annual average δ18O and annual average δ2H values versus time over the period 1990–2019 indicate increased gradients in both stable isotopes (≈0.3–0.5‰ and ≈1.1–3.2‰ per decade for δ18O and δ2H, respectively), accompanied with a decrease gradient of ≈0.9–1.1‰ per decade in d-excess values. Variability of annual 3H concentrations towards low levels (< 6 TU) during the later years strongly suggests the return back towards the cosmogenic production of this radioisotope in the upper atmosphere. Information gained from this work would offer new insights to improve the understanding of the temporal variability of P isotopes and assess the risks associated with climate change on the natural water resources in the Eastern Mediterranean region.

Formation pathways and source apportionments of inorganic nitrogen-containing aerosols in urban environment: Insights from nitrogen and oxygen isotopic compositions in Guangzhou, China

Air quality issues caused by PM2.5-induced persistent extreme haze episodes have become increasingly serious in urban environments in recent years. Secondary water-soluble inorganic sulfate (SO42−), nitrate (NO3−) and ammonium (NH4+) ions are the major components of PM2.5. However, the contributions of inorganic nitrogen species, especially nitrate, to PM2.5 have greatly increased during haze episodes in many cities in China. Therefore, better understanding of their emission sources and formation pathways holds the key to controlling urban PM2.5 pollution more efficiently and effectively. In this study, water-soluble ionic characteristics and isotopic compositions and sources of NH4+ and NO3−, as well as NO3− formation pathways were determined in PM2.5 aerosol samples collected in Guangzhou, China, during 2015–2018. The PM2.5 concentrations varied from 30.5 to 189.8 μg⋅m−3 and their mean values were highest in spring (111.1 μg⋅m−3) and lowest in summer (63.5 μg⋅m−3). The δ15N–NH4+ and δ15N–NO3- values ranged from +4.5 to +20.2‰ and from +4.8 to +14.8‰, respectively, with their mean values being highest in winter (14.0‰ and 9.5‰, respectively) and lowest in summer (10.4‰ and 7.1‰, respectively). The seasonal δ15N variability was mainly attributed to isotopic equilibrium fractionation, and partly due to the changes in NH3 and NOx sources. The average δ18O–NO3- value of 63.0‰, ranging seasonally from +58.6‰ in summer to +68.0‰ in spring, suggests that NO2 +⋅OH pathway played a vital role (70.3–85.9%) in NO3− formation. The Bayesian isotope mixing model results revealed fossil fuel combustion sources as dominant sources of atmospheric NH3 and NOx. This study suggests that more effort should be devoted to reduce NH3 and NOx from combustion-related processes and highlights the importance of δ18O–NO3- analysis for exploring variations of nitrate formation pathways in urban atmospheres.

Graphitic inclusions in zircon from early Phanerozoic S-type granite: Implications for the preservation of Hadean biosignatures

A biotic origin of isotopically light graphite in Hadean zircon remains contested, in part because it is unclear how biogenic carbon in sediments behaves during diagenesis, metamorphism and anatexis, and how it can be preserved as inclusions in zircon. Here, we report the discovery of graphitic inclusions in zircon from Rumburk granite in the Lusatian Block of eastern central Europe as the first example of such inclusion-bearing zircon in a well-constrained geological and petrological context. Most graphite inclusions are trapped at the interface between inherited zircon interiors of early Cambrian–Proterozoic age and late Cambrian (496.2 ± 2.3 Ma; 95% confidence) zircon overgrowths that crystallized at ∼700 °C under highly reducing conditions (typically −4.6 log units relative to the fayalite-magnetite-quartz buffer). Zircon overgrowths are also enriched in xenotime component (up to 93 µmol/g P) and δ18O (average δ18O = 7.7‰), both typical for S-type granitic melts. Raman microspectroscopy reveals crystalline and disordered graphitic carbon, but because overgrowth temperatures reached >700 °C, sufficient for complete graphitization, disordering is presumably secondary and caused by in-situ irradiation from the zircon host. Because organic materials were avoided during sample preparation, and inclusions were excavated by ion beam sputtering, exposure to potential C-bearing contaminants can be dismissed. Inclusions range in δ13C from −44.6 to −7.5‰ with a dominant mode at δ13C = −34‰ that is positively skewed. Rayleigh-type graphite precipitation from a CO2-CH4 fluid with a starting composition equivalent to regional black shale (δ13C = −32‰) explains inclusion carbon isotopic range and distribution. Collectively, these observations suggest that graphite was initially trapped in voids formed via dissolution-reprecipitation of trace element enriched, and possibly metamict, detrital zircon in metasedimentary protoliths when exposed to C-O-H fluids during prograde metamorphism. Subsequently, graphite-filled voids in inherited zircon became enclosed when zircon rims crystallized from highly reduced S-type granitic melts. Besides partial disordering of graphitic carbon due to irradiation, there is no indication for post-entrapment alteration, demonstrating that graphitic inclusions in zircon can preserve isotopic biosignatures over hundreds of millions of years.

Impact of soil salinity on groundwater chemistry in semi-arid regions in Western India: Insights from major ion and stable isotopic [formula omitted] characteristics.

Detailed geochemical and stable isotopic (δDH2O, δ18OH2O, and δ13CDIC) study of groundwater samples (n = 74) from a semi-arid region in Western India was carried out to constrain their solute sources and sub-surface weathering processes. Na+, Cl−, and HCO3− dominate major ion chemistry. This type of chemistry points to a significant solute supply from the salt-affected soils and bedrock dissolution. The average δ18O and δD in our groundwater samples are similar to their corresponding values in precipitation samples. Spatial variability of the δ18O and δD data depicts the impact of continental effect and variable extent of evaporation in this region. The δ18O-δD cross-plot yields a lower slope (5.2 ± 0.5) than that of the global meteoric water line (∼8), confirming the significant impact of evaporation on groundwater hydrology. δ13CDIC values vary between −7.3‰ and −16.4‰ (average = −9.7‰ ± 1.7‰), and first-order binary mixing calculation shows that about 75 (±16) % of the DIC in these samples are supplied via carbonate dissolution (range: 35–100%). Inverse model calculations involving elemental ratios of major ions show that the solutes are mainly supplied through salt-affected soils (30 ± 22%), with sub-ordinate contributions from rain (31 ± 13%), silicate (23 ± 12%) and carbonate (15 ± 13%). These estimates are consistent with the formation of salt-affected soils via evaporative water loss in semi-arid regions and the faster dissolution kinetics of these soil salts. The outcomes of this study underscore distinctly different compositions of semi-arid groundwater reservoirs, which has implications for drinking and irrigation usage.